In general, it is known that offset printing processes employ a cylinder which is covered with an offset plate receiving water and ink to form a latent image which is then transferred onto a blanket cylinder consisting of an outside lithographic layer capable of transferring the image onto a paper medium for example.
The transfers of water and ink from the offset plate to the lithographic layer, and then from the lithographic layer onto the paper are governed by a certain number of affinity parameters, to the water and the ink, of the offset plate, the lithographic layer of the blanket cylinder, and of the paper.
These parameters can be summarised in terms of a surface energy which can be broken down into a dispersive component and polar components.
In this respect, reference can be made to the following publication: R. J. Good, J. Adhesion Sci. Technol, Vol. 6, No. 12, 1269 (1992).
In short, the surface energies of a polar character, expressed in millijoules per square meter, and which are used to characterise the ability to transfer the ink and the water, are the following three components:
the polar component to water, which is used to describe the wetting potential by water and the wetting potential by the ink-water emulsion,
the polar component to formamide, which is used to express the basic character of the surface, and therefore its affinity with the acid wetting solutions, and
the polar component to dimethyl sulphoxide (DMSO), which is used to describe the acid value of the surface, and thus its affinity with inks which have a light basic polar composition.
In addition, the surface energy of a dispersive character is specified in terms of its dispersive component.
This being the case, if good ink transfer to the paper is required, then a good compromise must be found for the values of the above components, in order once again to ensure good transfer of the ink-water emulsion from the offset plate onto the lithographic layer of the blanket cylinder, and from the blanket cylinder to the paper.
Most of the known lithographic layers for blanket printing cylinders are made from nitrile rubber.
Such a layer constitutes a non-polar or weakly polar surface, so that it is slightly wetted by the water which is polar, and so that the ink tends to accumulate on the said surface. Thus the surface of the blanket cylinder gets dirty easily. Moreover, transfer of the ink to the paper is far from ideal, with the result that printing on paper can be unsatisfactory.
Now as one understands it, if the dispersive component of the lithographic layer is low, very little ink from the offset plate will be taken up by the said layer, and the printing process will be faulty.
On the other hand, if the dispersive component of the lithographic layer is high, then a large quantity of ink will be taken up by the blanket cylinder, but then its release onto the paper will be difficult, and the blanket cylinder will become dirty.
It will therefore be necessary to wash the blanket cylinder frequently, or even to replace it, not to mention that printing with such a blanket cylinder with a nitrile rubber lithographic layer will use a great deal of ink.